Atmospheric pressure is the pressure exerted by the Earth's atmosphere as a result primarily of the weight of the air above a given location and the momentum transferred by atmospheric molecules. At sea level under standard reference conditions, standard atmospheric pressure is defined as 101325 pascals, equivalent to 1013.25 hectopascals or approximately 1.01325 bar. Atmospheric pressure is not constant around the world or even at one location because it varies with altitude, temperature, humidity, weather systems and other atmospheric conditions.
A barometer measures atmospheric pressure, and meteorologists commonly express it in hectopascals or millibars, with one hectopascal equal to one millibar. Atmospheric pressure acts in all directions because gas molecules move randomly and collide with surfaces from many directions. The pressure measured at a location represents the combined effect of the atmosphere above and the local thermodynamic state of the air.
Atmospheric pressure can be understood through hydrostatic equilibrium. Air near Earth's surface supports the weight of the air above it, so pressure generally decreases as altitude increases. The pressure gradient is balanced by gravity in a stationary atmosphere. Although the atmosphere is constantly moving, the hydrostatic approximation works well for many large scale atmospheric calculations because vertical pressure adjustment occurs rapidly compared with many horizontal atmospheric processes.
At the microscopic level, the pressure results from molecular collisions, while at the macroscopic level the vertical pressure gradient balances the weight of the air column. The atmosphere therefore does not need a physical roof to produce pressure. The gravitational field keeps the gas concentrated near Earth's surface, and the accumulated mass of air produces substantial pressure at ground level.
Atmospheric pressure varies with weather because air masses differ in temperature, density and vertical structure. Regions of relatively high surface pressure are associated with atmospheric configurations that produce greater pressure at the surface, while low pressure systems involve different vertical mass distributions and commonly correspond with rising air and particular weather patterns. Pressure gradients drive horizontal winds because air accelerates in response to differences in pressure, although Earth's rotation introduces the Coriolis effect and friction modifies near surface motion.
Meteorologists therefore use pressure maps to analyse weather systems, fronts and wind patterns. Pressure measured at a station can be corrected to sea level for weather map comparisons, but such corrected values should not be confused with the actual pressure physically measured at high altitude.
Atmospheric pressure has numerous everyday physical effects. A suction cup can adhere to a surface because pressure is reduced inside the cup and the higher external atmospheric pressure produces a net force pressing it against the surface. Drinking through a straw works because reducing pressure inside the straw allows atmospheric pressure acting on the liquid surface to push liquid upward, rather than because the mouth directly pulls the liquid upward in the simplistic sense.
Syringes, vacuum pumps and some lifting devices similarly exploit pressure differences. Atmospheric pressure also affects boiling, evaporation and the operation of certain mechanical and pneumatic systems. The force exerted by atmospheric pressure can be enormous when applied over large areas, but objects are normally not crushed by it because pressure from gases and fluids inside the objects and the mechanical strength of materials provide balancing forces.
Atmospheric pressure is important in aviation and human physiology. As altitude increases, atmospheric pressure decreases, reducing the partial pressure of oxygen in the air. Aircraft cabins are therefore pressurised to provide conditions compatible with human physiology at cruising altitude. Climbers and high altitude workers experience lower ambient pressure and correspondingly lower oxygen partial pressure. The reduced pressure also affects boiling temperatures, gas expansion and breathing equipment.
Aviation altimeters exploit the relationship between atmospheric pressure and altitude, although accurate altitude determination requires calibration and atmospheric assumptions. Pressure changes can also affect the operation of pneumatic instruments and the performance of engines and other equipment because air density changes with pressure and temperature.
Barometers traditionally use mercury or an aneroid mechanism, while modern meteorological instruments commonly use electronic pressure sensors. A mercury barometer balances atmospheric pressure against the hydrostatic pressure of a mercury column. An aneroid barometer uses a sealed flexible chamber whose deformation changes as external pressure changes. Electronic sensors can measure pressure through changes in electrical properties of a sensing element. Atmospheric pressure measurements are useful in weather forecasting because pressure patterns change systematically with atmospheric circulation. A falling or rising pressure trend can provide information about approaching weather systems, although pressure alone cannot determine the complete weather state. Measurements must be interpreted alongside temperature, humidity, wind, cloud structure and other observations.
Atmospheric pressure is sometimes expressed in unusual units because of historical and engineering conventions. One standard atmosphere is 101325 pascals. Meteorology often uses hectopascals, while engineering may use pascals, kilopascals or bar. Inches of mercury remain common in some aviation and meteorological contexts. Pressure values in these units are convertible, but care is needed to distinguish absolute pressure from gauge pressure.
A tyre inflated to a specified gauge pressure contains a pressure above the surrounding atmospheric pressure, so its absolute internal pressure is the sum of gauge and local atmospheric pressure. This distinction becomes particularly important at high altitude because the same gauge pressure corresponds to a different absolute pressure relative to the surrounding environment.
Atmospheric pressure is therefore the macroscopic pressure field produced by Earth's atmosphere under gravity. Its magnitude reflects the mass and thermodynamic state of the air column above a location, while its spatial and temporal variation reflects atmospheric dynamics. It affects weather, aviation, fluid behaviour, engineering systems, biological processes and everyday devices. Standard sea level pressure is a reference rather than a universal constant: actual atmospheric pressure varies continuously. Understanding atmospheric pressure requires connecting molecular kinetic theory with hydrostatic balance, atmospheric composition, temperature, gravity and large scale circulation.
